The invention relates to capacitive position sensors. More particularly the invention relates to 2-dimensional capacitive position sensors of the type based on capacitive proximity sensing techniques. Such sensors may be referred to as 2-dimensional capacitive transducing (2DCT) sensors. 2DCT sensors are based on detecting a disturbance in a capacitive coupling of sensor electrodes caused by the proximity of a pointing object. A measured location for the disturbance corresponds to a measured position for the pointing object.
U.S. Pat. No. 6,452,514, U.S. Pat. No. 7,148,704 and U.S. Pat. No. 5,730,165 disclose prior art capacitive touch sensors.
2DCT sensors are typically actuated by a human finger, or a stylus. Example devices include touch screen and touch sensitive keyboards/keypads, e.g. as used for controlling consumer electronic devices/domestic appliances, and possibly in conjunction with an underlying display, such as a liquid crystal display (LCD), or cathode ray tube (CRT). Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example. 2DCT sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
Devices employing 2DCT sensors have become increasingly popular and common, not only in conjunction with personal computers, but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments, or where there is a danger of dirt or fluids entering a device being controlled attractive. Furthermore still, manufacturers often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. push-buttons).
WO 2009/027629, published on 5 Mar. 2009, describes a capacitive touch sensor comprising a dielectric panel overlying a drive electrode with two sense electrodes. One of the sense electrodes is positioned to be shielded from the drive electrode by the first sense electrode, so that the first sense electrode receives the majority of the charge coupled from the drive electrode and the second sense electrode primarily registers noise. A sensing circuit including two detector channels is connected to the first (coupled) and second (noise) sense electrodes to receive signal samples respectively. The sensing circuit is operable to output a final signal obtained by subtracting the second signal sample from the first signal sample to cancel noise.
However, the methods described above increase the size and thickness, and may decrease the resolution of a device incorporating a display screen with a 2DCT sensor when it is more fashionable and desirable to produce smaller devices. Furthermore, additional steps are required during manufacture and as a result there is an increased cost due to further components being needed.
European patent EP 1821175 describes an alternative solution to reduce the noise collected on a 2DCT touch sensor. EP 1821175 discloses a display device with a touch sensor which is arranged so that the two dimensional touch sensor is overlaid upon a display device to form a touch sensitive display screen. The display device uses an LCD arrangement with vertical and horizontal switching of the LCD pixels. The touch sensing circuit includes a current detection circuit, a noise elimination circuit as well as a sampling circuit for each of a plurality of sensors, which are arranged to form the two-dimensional sensor array. The current detection circuit receives a strobe signal, which is generated from the horizontal and vertical switching signals of the LCD screen. The strobe signal is used to trigger a blanking of the current detection circuit during a period in which the horizontal switching voltage signal may affect the measurements performed by the detection circuit.
WO 2009/016382, published on 5 Feb. 2009, describes a sensor used to form a two dimensional touch sensor, which can be overlaid on a liquid crystal display (LCD) screen. As such, the effects of switching noise on the detection of an object caused by a common voltage signal of the LCD screen can be reduced. The sensor comprises a capacitance measurement circuit operable to measure the capacitance of the sensing element and a controller circuit to control charging cycles of the capacitance measurement circuit. The controller circuit is configured to produce charging cycles at a predetermined time and in a synchronous manner with a noise signal. For example, the charge-transfer cycles or ‘bursts’ may be performed during certain stages of the noise output signal from the display screen, i.e. at stages where noise does not significantly affect the capacitance measurements performed. Thus, the sensor can be arranged to effectively pick up the noise output from a display screen and automatically synchronize the charge-transfer bursts to occur during stages of the noise output cycle.
FIG. 21 of the accompanying drawings illustrates schematically a representative portion of the prior art electrode pattern of U.S. Pat. No. 6,452,514 or its equivalent WO 00/44018, published on 27 Jul. 2000. A plurality of drive electrodes X1, X2, X3 and X4 extending rowwise are arranged with a plurality of sense electrodes Y1, Y2, Y3 and Y4 extending columnwise, the intersections or crossings between X and Y electrodes forming a matrix or grid of sensing points or areas 220. It will be understood the X and Y electrodes do not literally intersect, but are offset in the vertical or Z direction, orthogonal to the plane of the drawing, being separated by a dielectric layer—typically a substrate panel which bears the X electrodes on one side and the Y electrodes on the other side. Each crossed electrode area 220 acts as a key so that the presence of a body such as a user's finger is detected as a result of a change in an amount of charge which is transferred between the two electrodes at the key location. With this arrangement, each of the electrodes X1, X2, X3 and X4 are driven with a drive circuit 118 via connections 105 and the other electrodes Y1, Y2, Y3 and Y4 are connected to a charge measurement circuit 118 via sense channels 116 which detects an amount of charge present at each of the sensing areas 220. It will be appreciated that for simplicity all of the control circuitry has been included in a single circuit 118. Such two dimensional capacitive transducing (2DCT) sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in for example in consumer electronic devices and domestic appliances. The 2DCT is of the so-called “active” or “mutual” type, in which proximity of an object is sensed by the changes induced in coupling between a drive electrode and one or more adjacent sense electrodes.
In the above 2DCT sensor, interpolation is used to determine the location of an object or finger adjacent the sensor. This is done by using the signals from the sense area being touched and the neighboring sense areas in a linear interpolation algorithm. However, for an interpolation to be accurate the electric field between adjacent drive electrodes should be linear or at least known. If the electrodes are placed close together it can be assumed that the electric filed between two electrodes is linear. That is to say that as you move away from an electrode, the field reduces in a linear fashion.
As the size of devices that use 2DCT sensors is increased, larger area 2DCT sensors are required. To increase the area of the 2DCT sensor while keeping the same resolution and accuracy (i.e. avoid using a non-linear interpolation method) the number of drive and sense electrodes could be increased. However, this means that the number of connections required from the control circuits is increased which in turn results in more expensive control circuits and increased acquisition times, since the acquisition of signals from each of the sensing areas typically needs to be carried out at least partially in series, since not all sensing areas can be polled simultaneously owing to restrictions on the number of drive and sense lines, and controller channels, i.e. chip pins.
FIG. 22 of the accompanying drawings illustrates schematically a representative portion of the prior art electrode pattern US 2008/0246496, published on 9 Oct. 2008. The figure illustrates a pattern of electrodes comprising longitudinal (bar) drive electrodes 152. The drive electrodes 152 are coupled via drive channels 158 and 160 to a controller (not shown in the figure). Each drive channel supplies drive signals to the group of four drive electrodes 152. The drive electrodes 152 are each connected to one another by a chain or row of resistors 170 having the same value. Alternatively, a single resistive strip could be used (not shown in figure). When operated the grouped drive electrodes will receive a different value drive signal. For example, when drive channel 160 is connected to a drive signal and drive channel 158 is connected to ground, the electrode connected directly to drive channel 160 will receive the applied signal value, the drive electrode below will receive two thirds of the applied signal value and the drive electrode below that will receive a third of the applied signal value. In the example described above, the fourth electrode connected directly to the drive channel 158 in the figure will be connected to ground. However, the above method can be repeated with drive channel 158 being connected to a drive signal and drive channel 160 being connected to ground. This effectively, allows four drive electrodes to be driven using only two drive channels. The arrangement shown in the figure can be repeated, and expanded to include more intermediate drive electrodes with respective resistors. However, the method described above is only suitable for the drive electrodes and is not transferable to the sense electrodes. The sense electrodes shown in the figure are interleaved with adjacent drive electrodes on a single surface. However, it will be appreciated that the drive electrodes shown in the figure could also be used for two-layer or dual layer designs.
It would therefore be desirable to provide an electrode pattern for a mutual capacitive or active type 2DCT sensor that can be used to allow the size of the overall sensitive area to be increased without needing to introduce more sense channels.